Photosensitive material such as silver salts have been employed as recording materials utilizing a light beam of a high energy density. Thermal recording materials have also been used for the same purposes. In such thermal recording materials, a recording layer has a high optical density. The recording layer absorbs an irradiated light beam of a high energy density causing a local temperature rise. Accordingly the irradiated area will undergo deformation by heat, such as melting-cohesion or evaporation. As a result, the area which has been irradiated by light is removed producing a difference in optical density from the non-irradiated area. Thereby information is recorded (cf. U.S. Pat. Nos. 4,188,214, 4,291,119, 4,216,501, 4,233,626, 4,188,214 and 4,291,119 and British Pat. No. 2,026,346). Such thermal recording materials are generally desirable with respect to processing. Conventional development and fixing are not necessary. Furthermore, since the recording layer is not sensitive to ordinary indoor light, operations in a darkroom are not required. In addition an image having a high contrast can be obtained, and it is possible to add more information (i.e. add-on).
In general, the method for recording on such thermal recording materials is often achieved by converting the information to be recorded into an electrical time sequence signal, and then scanning with a laser beam. The intensity of the laser beam has been modulated with respect to a signal, on the recording material. This is desirable because a recorded image can be obtained in actual time, i.e. instantaneously.
Suitable examples of the recording layer for such thermal recording materials are metals, dyestuffs and plastics. In general, an inexpensive material is used. Such recording materials are described in, for example, M. L. Levene et al, "Electron, Ion and Laser Beam Technology", the paper submitted to the 11th Symposium (1969), Electronics (Mar. 18, 1968), p. 50; D. Maydan, "The Bell System Technical Journal", Vol. 50 (1971), p. 1761; C. O. Carlson, "Science", Vol. 154 (1966), p. 1550 etc. Among the above, examples in which a metal is employed in the recording layer are those in which a thin film of a metal such as Bi, Sn, In etc. is used on a base. These metals have excellent properties as thermal recording materials, since an image of a high resolving power can be recorded. More specifically, can be used to obtain a high contrast image.
However, in general, recording materials utilizing metal thin films are not desirable because most of them have a light reflectance of 50% or higher with respect to laser light employed for recording. Therefore, the energy of the laser light can not be efficiently utilized. Since a large amount of light energy is necessary for recording a laser source of a great output is necessary for recording by high-speed scanning. As a result, a large expensive recording apparatus is inevitably required. In view of these circumstances recording materials having high recording sensitivity have been investigated. For instance, recording materials comprised of three layers of Se, Bi and Ge are described in Japanese patent publication No. 40479/1971. In that patent publication, the Se layer is provided for reducing the reflectance by the Bi layer for the irradiated light. The Se layer is an easily evaporable layer, so that both of them promote thermal deformation of the Bi layer, the main layer, by a small amount of energy as compared with the case of the Bi layer alone. The layers for reducing or preventing reflectance are also described in Japanese patent application (OPI) 151151/1975, (the term "OPI" as used herein refers to a "Published unexamined Japanese patent application") and Japanese patent publication No. 14262/1976. In addition, materials where a layer for reducing thermal conductivity is provided between a recording layer and its support are disclosed in Japanese patent application (OPI) No. 126237/1976 and Japanese patent application (OPI) No. 16026/1976. Still further, Japanese patent application (OPI) No. 78236/1976 (corresponding to U.S. Pat. Nos. 4,188,214 and 4,291,119) and Japanese patent application (OPI) No. 20821/1977 describe recording layers in which a metal sulfide, metal fluoride or metal oxide is overlaid with or mixed with a metal.
Information is recorded on the recording materials described above by irradiating with a beam of high energy density, such as a laser beam. This causes thermal deformation of the recording layer, completely removing the entire irradiated area of the recording layer. Therefore, in such a recording mode, it is essential that the energy of the laser beam be high enough to remove the entire irradiated area of the recording layer. For this reason, a laser light of considerably high energy is still required even when using a means such as the above-described sensitization. Reading of the record thus obtained is conducted by transmitted light, because if reading is made by the reflected light, only a small range of lights (determined by the composition of the recording layer) can be employed. Furthermore, the incident angle to the recording layer is also restricted to an extremely narrow range.